bpo-31970: Reduce performance overhead of asyncio debug mode.

Lib/asyncio/coroutines.py

@@ -91,7 +91,7 @@ def __init__(self, gen, func=None):
  assert inspect.isgenerator(gen) or inspect.iscoroutine(gen), gen
  self.gen = gen
  self.func = func # Used to unwrap @coroutine decorator
- self._source_traceback = traceback.extract_stack(sys._getframe(1))
+ self._source_traceback = events.extract_stack(sys._getframe(1))
  self.__name__ = getattr(gen, '__name__', None)
  self.__qualname__ = getattr(gen, '__qualname__', None)
 

Lib/asyncio/events.py

@@ -72,6 +72,24 @@ def _format_callback_source(func, args):
  return func_repr
 
 
+def extract_stack(f=None, limit=None):
+ """Replacement for traceback.extract_stack() that only does the
+ necessary work for asyncio debug mode.
+ """
+ if f is None:
+ f = sys._getframe().f_back
+ if limit is None:
+ # This is heuristically the max number of entries we're gonna pop + 1
+ # (we're only interested in displaying the top of stack)
+ # If this is too small, tests will fail.
+ limit = 4
+ stack = traceback.StackSummary.extract(traceback.walk_stack(f),
+ limit=limit,
+ lookup_lines=False)
+ stack.reverse()
+ return stack
+
+
 class Handle:
  """Object returned by callback registration methods."""
 
@@ -85,7 +103,7 @@ def __init__(self, callback, args, loop):
  self._cancelled = False
  self._repr = None
  if self._loop.get_debug():
- self._source_traceback = traceback.extract_stack(sys._getframe(1))
+ self._source_traceback = extract_stack(sys._getframe(1))
  else:
  self._source_traceback = None
 

Lib/asyncio/futures.py

@@ -77,7 +77,7 @@ def __init__(self, *, loop=None):
  self._loop = loop
  self._callbacks = []
  if self._loop.get_debug():
- self._source_traceback = traceback.extract_stack(sys._getframe(1))
+ self._source_traceback = events.extract_stack(sys._getframe(1))
 
  _repr_info = base_futures._future_repr_info
 

Misc/NEWS.d/next/Library/2017-11-07-14-20-09.bpo-31970.x4EN_9.rst

@@ -0,0 +1 @@
+Reduce performance overhead of asyncio debug mode.